Nolvadex blocks estrogen at some receptors and acts as an estrogen at other receptors while arimidex prevents the formation of estrogen thereby reducing circulating estrogen levels.
If you're using arimidex there's no need for nolvadex as a anti-estrogen. However, there is some evidence that it can improve lipid profiles (due to its weak estrogenic effect) while using an aromatase inhibitor like arimidex. That's the only reason I can think of for using both at the same time.